Many genetic diseases affect the nervous system. The long term goal of this grant proposal is to develop avirulent herpes simplex virus type 1 (HSV-1) strains as gene therapy vectors for nervous system disorders. There are two characteristics of HSV-1 infections that will be exploited to achieve this goal: HSV-1 establishes life-long latent infections in neurons, and during latent infection only a single viral promoter is active. This promoter directs the synthesis of the HSV-1 latency associated transcripts (LATs), which are expressed for the lifetime of the latently infected individual. To demonstrate the feasibility of HSV-1- mediated gene therapy, a cDNA for the human hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene will be placed under the control of the HSV-1 LAT gene promoter. A complete deficiency of HPRT causes Lesch- Nyhan syndrome, a severe and untreatable neurological disease. Since individuals with very low HPRT levels are spared from neurological dysfunction, even partial replacement of HPRT in the nervous system may be therapeutic. HPRT-negative transgenic mice are available as an animal model of Lesch-Nyhan syndrome to assess the effectiveness of gene transfer techniques. There are several avirulent HSV-1 strains available, both replication competent and incompetent. These viruses do not produce any disease in mice, even when inoculated intracerebrally (k.c.). To develop HSV-1 for gene delivery to the nervous system, we will i) study several avirulent HSV-1 strains for spread through the nervous system, cytopathology, viral RNA and protein expression during acute infection, and the type, number and distribution of LAT expressing cells during latency, ii)determine the polyA+/regulatory sequence requirements for high levels of HSV-1 latency promoter directed cDNA expression in the most promising avirulent strains(s), and iii) investigate the tissue distribution of human HPRT mRNA and proteins in normal mice and in HPRT- negative transgenic mice infected with the HSV-1/HPRT recombinants.